Imines and Their N -Substituted Derivatives: Oximes and Their O -R Substituted Analogs

Oximes are useful as protected aldehydes and ketones, and as intermediates for rearrangement and cycloaddition reactions. Efficient methods of synthesis from carbonyl compounds and hydroxylamines are described, including recent developments in green chemistry approaches. Alternatively, oximes can be synthesized from noncarbonyl compounds such as nitronates or nitro compounds. Approaches to the synthesis of α,β-unsaturated oximes and cyclic oximes are discussed. Oxime ethers are accessible through a number of routes, such as multicomponent radical coupling or oxidation of alcohols in the presence of alkoxylamines. Nitrones, a class of extremely useful synthetic intermediates, may be synthesized from a variety of precursors. In addition to synthesis from aldehydes and ketones, other routes to nitrones include direct oxidation of hydroxylamines or amines, and reduction of nitroalkanes. The synthesis of enantiomerically pure N-substituted imines, including the sulfenimines, sulfinimines, and sulfonimines, has been of great interest in recent years. They are particularly useful in the asymmetric synthesis of important functional groups such as aziridines and α-amino acids. N-Phosphinoyl and N-silylimines (used as masked imines) are also synthetically useful intermediates.